Steering control apparatus

ABSTRACT

Provided is a steering control apparatus in which the response of steering assistance to driver&#39;s steering can be improved during execution of rotation angle sensor-less control. When the rotation angle sensor-less control is executed and when an induced voltage value is equal to or lower than a threshold voltage, driving of a motor is controlled based on a second addition angle. The second addition angle is calculated such that values obtained by multiplying a first pre-addition angle calculated based on a steering torque and a second pre-addition angle calculated based on an induced voltage derivative by respective predetermined use ratios that are based on the induced voltage value are added together. By using the second addition angle, a more appropriate estimated electrical angle is calculated in response to a driver&#39;s steering situation.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2017-215015 filed onNov. 7, 2017 including the specification, drawings and abstract, isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a steering control apparatus.

2. Description of the Related Art

Hitherto, there exists an electric power steering system (EPS)configured to apply a torque of a motor to a steering mechanism of avehicle as an assist force. As described in, for example, JapanesePatent Application Publication No. 2014-138530 (JP 2014-138530 A), acontrol apparatus of the EPS controls driving of the motor by using anelectrical angle of the motor that is detected through a rotation anglesensor. When any abnormality occurs in the rotation angle sensor, thecontrol apparatus executes so-called rotation angle sensor-less controlfor controlling the driving of the motor by using an estimatedelectrical angle that is estimated based on an induced voltage(counter-electromotive voltage) generated in the motor in place of theelectrical angle that is based on a detection result from the rotationangle sensor. The control apparatus calculates an addition angle(electrical angle by which the motor rotates in one calculation period)based on the induced voltage, and calculates the estimated electricalangle by integrating the addition angle. The positive or negative signof the addition angle is determined by, for example, a rotationdirection of the motor that is estimated based on the positive ornegative sign of a steering torque.

There is a proportional relationship between the magnitude of the valueof the induced voltage generated in the motor and an angular velocitythat is a change amount of the motor per unit time. Therefore, the valueof the induced voltage decreases as the angular velocity of the motordecreases. In this case, it may be difficult to secure the calculationaccuracy of the estimated electrical angle and furthermore secure theresponse of steering assistance to driver's steering because theinfluence of noise is likely to increase, for example.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide a steering controlapparatus in which the response of steering assistance to driver'ssteering can be improved during execution of rotation angle sensor-lesscontrol.

One aspect of the present invention relates to a steering controlapparatus configured to calculate a current command value for a motorbased on at least a steering torque, calculate an estimated electricalangle of the motor based on an induced voltage generated in the motor,and control power supply to the motor by using the calculated estimatedelectrical angle. The motor is a source of power to be applied to asteering mechanism of a vehicle.

The steering control apparatus includes a first estimated electricalangle calculation circuit, a second estimated electrical anglecalculation circuit, a selection circuit, and an integration circuit.The first estimated electrical angle calculation circuit is configuredto calculate, based on the induced voltage, a first addition angle thatis a change amount of the estimated electrical angle in one calculationperiod. The second estimated electrical angle calculation circuit isconfigured to calculate, based on the steering torque, a second additionangle that is a change amount of the estimated electrical angle in onecalculation period. The selection circuit is configured to select thefirst addition angle when the induced voltage is higher than a thresholdvoltage, and select the second addition angle when the induced voltageis equal to or lower than the threshold voltage. The integration circuitis configured to calculate the estimated electrical angle by integratingthe first addition angle or the second addition angle that is selectedby the selection circuit.

According to the configuration described above, when the induced voltagegenerated in the motor is equal to or lower than the threshold voltage,the estimated electrical angle is calculated based on the secondaddition angle that is based on the steering torque. The phase of thesecond addition angle is compensated based on the derivative of thesteering condition amount (state variable). The phase of the derivativeof the steering condition amount is advanced relative to that of thesteering condition amount. Therefore, the phase of the second additionangle is also advanced as a whole by compensating the phase of thesecond addition angle based on the derivative of the steering conditionamount. By using the second addition angle, a more appropriate estimatedelectrical angle is calculated in response to the driver's steeringsituation. Thus, the response of the steering assistance to the driver'ssteering can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further features and advantages of the invention willbecome apparent from the following description of example embodimentswith reference to the accompanying drawings, wherein like numerals areused to represent like elements and wherein:

FIG. 1 is a configuration diagram illustrating an overview of anelectric power steering system on which a steering control apparatus ofone embodiment is mounted;

FIG. 2 is a block diagram illustrating the electrical configuration ofthe electric power steering system of the embodiment;

FIG. 3 is a functional block diagram of a microcomputer of the steeringcontrol apparatus of the embodiment;

FIG. 4 is a functional block diagram of a rotation angle estimationcircuit of the steering control apparatus of the embodiment;

FIG. 5 is a functional block diagram of a second estimated electricalangle calculation circuit of the rotation angle estimation circuit ofthe embodiment;

FIG. 6 is a graph illustrating a first map to be used in the secondestimated electrical angle calculation circuit of the embodiment;

FIG. 7 is a graph illustrating a second map to be used in the secondestimated electrical angle calculation circuit of the embodiment; and

FIG. 8 is a graph illustrating a third map to be used in the secondestimated electrical angle calculation circuit of the embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

A steering control apparatus of one embodiment of the present inventionis described below. As illustrated in FIG. 1, an electric power steeringsystem 1 includes a steering mechanism 2 and an assist mechanism 3. Thesteering mechanism 2 turns steered wheels 15 based on a driver'soperation for a steering wheel 10. The assist mechanism 3 assists thedriver's steering operation. A steering control apparatus 50 is mountedon the electric power steering system 1.

The steering mechanism 2 includes the steering wheel 10 and a steeringshaft 11. The steering shaft 11 is fixed to the steering wheel 10. Thesteering shaft 11 includes a column shaft 11 a, an intermediate shaft 11b, and a pinion shaft 11 c. The column shaft 11 a is coupled to thesteering wheel 10. The intermediate shaft 11 b is coupled to the lowerend of the column shaft 11 a. The pinion shaft 11 c is coupled to thelower end of the intermediate shaft 11 b. The lower end of the pinionshaft 11 c is coupled to a rack shaft 12 via a rack and pinion mechanism13. The right and left steered wheels 15 are coupled to both ends of therack shaft 12 via tie rods 14, respectively. Thus, rotational motion ofthe steering wheel 10, that is, the steering shaft 11 is converted toreciprocating linear motion of the rack shaft 12 in its axial direction(lateral direction in FIG. 1) via the rack and pinion mechanism 13constituted by the pinion shaft 11 c and the rack shaft 12. Thereciprocating linear motion is transmitted to the right and left steeredwheels 15 and 15 via the tie rods 14 coupled to the respective ends ofthe rack shaft 12. Thus, steered angles θt of the steered wheels 15 and15 are changed.

The assist mechanism 3 includes a motor 40. The motor 40 is a source ofpower (assist force) to be applied to the steering mechanism 2. Examplesof the motor 40 to be employed include a three-phase brushless motorconfigured to rotate based on three-phase (U, V, and W) driving electricpower. A rotation shaft 41 of the motor 40 is coupled to the columnshaft 11 a via a speed reducing mechanism 42. The speed reducingmechanism 42 reduces the speed of rotation of the motor 40 (rotationshaft 41), and transmits, to the column shaft 11 a, a rotational forceof the motor 40 that is obtained through the speed reduction. Therotational force transmitted to the column shaft 11 a is converted to anaxial force of the rack shaft 12 via the rack and pinion mechanism 13.The driver's steering operation is assisted by applying the convertedforce to the rack shaft 12 as the assist force.

The steering control apparatus 50 controls driving of the motor 40 basedon detection results from various sensors. Examples of various sensorsinclude a torque sensor 60, a rotation angle sensor 61, and a vehiclespeed sensor 62. The torque sensor 60 is provided on the column shaft 11a. The torque sensor 60 detects a steering torque Trq applied to thesteering shaft 11 through the driver's steering operation. The rotationangle sensor 61 is provided on the motor 40. The rotation angle sensor61 detects an electrical angle (rotation angle) θma of the motor 40. Thevehicle speed sensor 62 detects a vehicle speed V that is a travelingspeed of a vehicle.

Next, the electrical configuration of the steering control apparatus 50is described. As illustrated in FIG. 2, the steering control apparatus50 includes a microcomputer 51 and a drive circuit 52. The microcomputer51 generates a motor control signal for controlling the driving of themotor 40. The drive circuit 52 supplies a current to the motor 40 basedon the motor control signal generated by the microcomputer 51. The drivecircuit 52 and the motor 40 are connected together by power supply linesW1 u to W1 w. Current sensors 53 u, 53 v, and 53 w are provided on thepower supply lines W1 u to W1 w, respectively. The microcomputer 51 andthe drive circuit 52 are connected together by signal lines W2 u, W2 v,and W2 w. Voltage sensors 54 u, 54 v, and 54 w are provided on thesignal lines W2 u, W2 v, and W2 w, respectively. The voltage sensors 54u to 54 w divide terminal voltages of respective phases of the motor 40through voltage division resistors R1 and R2, and generate detectionsignals Su to Sw based on values obtained through the voltage division.

The microcomputer 51 acquires detection results from the torque sensor60, the rotation angle sensor 61, and the vehicle speed sensor 62 (Trq,θma, and V). The microcomputer 51 also acquires detection results fromthe current sensors 53 u, 53 v, and 53 w (Iu, Iv, and Iw) and detectionresults from the voltage sensors 54 u, 54 v, and 54 w (detection signalsSu, Sv, and Sw). Based on the acquired detection results, themicrocomputer 51 generates pulse width modulation (PWM) drive signals α1to α6 as the motor control signal.

The PWM drive signals α1 to α6 are signals for causing the drive circuit52 to execute PWM drive.

The drive circuit 52 is a PWM inverter circuit configured to convert adirect current (DC) voltage from a DC power supply (power supply voltage“+Vcc”) such as an on-board battery to an alternating current (AC)voltage and supply the AC voltage to the motor 40. The drive circuit 52is formed such that three sets of switching arms each having twoswitching elements connected in series are connected in parallel.Switching elements T1 and T2 constitute a switching arm corresponding tothe U phase. Switching elements T3 and T4 constitute a switching armcorresponding to the V phase. Switching elements T5 and T6 constitute aswitching arm corresponding to the W phase. The switching elements T1,T3, and T5 are provided on the power supply side, and the switchingelements T2, T4, and T6 are provided on a ground side.

A middle point Pu between the switching element T1 and the switchingelement T2, a middle point Pv between the switching element T3 and theswitching element T4, and a middle point Pw between the switchingelement T5 and the switching element T6 are connected to coils of therespective phases of the motor 40 via the power supply lines W1 u to W1w. By switching ON/OFF of the switching elements T1 to T6 based on thePWM drive signals α1 to α6 generated by the microcomputer 51, the DCvoltage supplied from the DC power supply is converted to three-phase(U-phase, V-phase, and W-phase) AC voltages. The three-phase AC voltagesobtained through the conversion are supplied to the coils of therespective phases of the motor 40 via the power supply lines W1 u to W1w, thereby driving the motor 40.

Next, the microcomputer 51 is described in detail. As illustrated inFIG. 3, the microcomputer 51 includes a current command valuecalculation circuit 70 and a control signal generation circuit 71.

The current command value calculation circuit 70 calculates currentcommand values. The current command value is a target value of a currentamount corresponding to an assist force to be generated in the motor 40.The current command value calculation circuit 70 acquires the vehiclespeed V and the steering torque Trq. The current command valuecalculation circuit 70 calculates a q-axis current command value Iq* anda d-axis current command value Id* based on the vehicle speed V and thesteering torque Trq. The q-axis current command value Iq* is a currentcommand value on a q-axis in a d/q coordinate system. The d-axis currentcommand value Id* is a current command value on a d-axis in the d/qcoordinate system. The current command value calculation circuit 70calculates a q-axis current command value Iq* having a higher absolutevalue as the absolute value of the steering torque Trq increases and asthe value of the vehicle speed V decreases. In this example, the currentcommand value calculation circuit 70 fixes the d-axis current commandvalue Id* to zero.

The control signal generation circuit 71 generates the PWM drive signalsα1 to α6 corresponding to the current command values. The control signalgeneration circuit 71 acquires the current command values (Iq* and Id*),the current values Iu, Iv, and Iw of the respective phases, and theelectrical angle θma. The control signal generation circuit 71 generatesthe PWM drive signals α1 to α6 through execution of current feedbackcontrol based on the current values Iu, Iv, and Iw of the respectivephases and the electrical angle θma so that actual current values of themotor 40 (q-axis current value and d-axis current value) follow thecurrent command values (Iq* and Id*).

In the control signal generation circuit 71, an estimated electricalangle θmb calculated by a rotation angle estimation circuit 77 describedlater may be used in place of the electrical angle θma detected throughthe rotation angle sensor 61.

The control signal generation circuit 71 includes a d/q conversioncircuit 72, a feedback control circuit (hereinafter referred to as “FBcontrol circuit”) 73, a d/q inversion circuit 74, and a PWM conversioncircuit 75. The d/q conversion circuit 72 acquires the current valuesIu, Iv, and Iw of the respective phases and the electrical angle θma.The d/q conversion circuit 72 calculates a d-axis current value Id and aq-axis current value Iq by mapping the current values Iu, Iv, and Iw ofthe respective phases on the d/q coordinates based on the electricalangle θma. The d-axis current value Id and the q-axis current value Iqare actual current values of the motor 40 in the d/q coordinate system.

The F/B control circuit 73 acquires a d-axis current deviation ΔId and aq-axis current deviation ΔIq. The d-axis current deviation ΔId isobtained by subtracting the d-axis current value Id from the d-axiscurrent command value Id*. The q-axis current deviation ΔIq is obtainedby subtracting the q-axis current value Iq from the q-axis currentcommand value Iq*. The F/B control circuit 73 calculates a d-axisvoltage command value Vd* by executing current feedback control based onthe d-axis current deviation ΔId so that the d-axis current value Idfollows the d-axis current command value Id*. The F/B control circuit 73calculates a q-axis voltage command value Vq* by executing currentfeedback control based on the q-axis current deviation ΔIq so that theq-axis current value Iq follows the q-axis current command value Iq*.

The d/q inversion circuit 74 acquires the d-axis voltage command valueVd*, the q-axis voltage command value Vq*, and the electrical angle θma.The d/q inversion circuit 74 calculates voltage command values Vu*, Vv*,and Vw* of the respective phases in a three-phase AC coordinate systemby mapping the d-axis voltage command value Vd* and the q-axis voltagecommand value Vq* on the three-phase AC coordinate system based on theelectrical angle θma.

The PWM conversion circuit 75 acquires the voltage command values Vu*,Vv*, and Vw* of the respective phases. The PWM conversion circuit 75generates the PWM drive signals α1 to α6 by executing PWM conversion forthe voltage command values Vu*, Vv*, and Vw* of the respective phases.The PWM drive signals α1 to α6 are applied to gate terminals of thecorresponding switching elements T1 to T6 of the drive circuit 52.

When an abnormality occurs in the rotation angle sensor 61 for somereason and the electrical angle θma cannot be detected properly, it maybe difficult to control the motor 40 appropriately. In this example,so-called rotation angle sensor-less control is executed as backupcontrol when an abnormality occurs in the rotation angle sensor 61. Thatis, the microcomputer 51 estimates an electrical angle based on aninduced voltage (counter-electromotive force) generated in the motor 40,and continuously controls the motor 40 by using the estimated electricalangle.

As illustrated in FIG. 3, the microcomputer 51 includes a terminalvoltage value calculation circuit 76, the rotation angle estimationcircuit 77, an abnormality detection circuit 78, and a rotation angleselection circuit 79 as components for executing the rotation anglesensor-less control.

The terminal voltage value calculation circuit 76 acquires the detectionsignals Su, Sv, and Sw that are the detection results from the voltagesensors 54 u, 54 v, and 54 w, respectively. The terminal voltage valuecalculation circuit 76 calculates terminal voltage values Vu, Vv, and Vwof the respective phases of the motor 40 based on the detection signalsSu, Sv, and Sw.

The rotation angle estimation circuit 77 acquires the terminal voltagevalues Vu, Vv, and Vw of the respective phases, the steering torque Trq,and the current values Iu, Iv, and Iw of the respective phases. Therotation angle estimation circuit 77 calculates the estimated electricalangle θmb based on the terminal voltage values Vu, Vv, and Vw of therespective phases, the steering torque Trq, and the current values Iu,Iv, and Iw of the respective phases.

The abnormality detection circuit 78 acquires the electrical angle θma.The abnormality detection circuit 78 detects an abnormality of therotation angle sensor 61 based on the electrical angle θma. For example,the abnormality detection circuit 78 detects the abnormality of therotation angle sensor 61 when the absolute value of a difference betweena present value and a previous value of the electrical angle θmadeviates from a predetermined permissible range. The permissible rangeis set in consideration of a control period of the microcomputer 51 or adetection tolerance of the rotation angle sensor 61. The abnormalitydetection circuit 78 generates an abnormality detection signal Se basedon a detection result. The abnormality detection signal Se includesinformation indicating the presence or absence of the abnormality of therotation angle sensor 61.

The rotation angle selection circuit 79 acquires the estimatedelectrical angle θmb calculated by the rotation angle estimation circuit77, the abnormality detection signal Se generated by the abnormalitydetection circuit 78, and the electrical angle θma. When the abnormalitydetection signal Se indicates that no abnormality occurs in the rotationangle sensor 61, the rotation angle selection circuit 79 selects, as amotor control electrical angle, the electrical angle θma that is adetection result from the rotation angle sensor 61. When the abnormalitydetection signal Se indicates that an abnormality occurs in the rotationangle sensor 61, the rotation angle selection circuit 79 selects, as themotor control electrical angle, the estimated electrical angle θmbcalculated by the rotation angle estimation circuit 77.

Next, the rotation angle estimation circuit 77 is described in detail.As illustrated in FIG. 4, the rotation angle estimation circuit 77includes a phase induced voltage value calculation circuit 84, aninduced voltage value calculation circuit 85, an angular velocitycalculation circuit 86, a first estimated electrical angle calculationcircuit 80, a second estimated electrical angle calculation circuit 81,a switching circuit 82 serving as a selection circuit, and anintegration circuit 83.

The phase induced voltage value calculation circuit 84 acquires thecurrent values Iu, Iv, and Iw of the respective phases and the terminalvoltage values Vu, Vv, and Vw of the respective phases. The phaseinduced voltage value calculation circuit 84 calculates induced voltagevalues eu, ev, and ew of the respective phases in the three-phase ACcoordinate system based on the current values Iu to Iw of the respectivephases and the terminal voltage values Vu, Vv, and Vw of the respectivephases. The phase induced voltage value calculation circuit 84 maycalculate the induced voltage values eu, ev, and ew of the respectivephases in consideration of resistance values of the coils of therespective phases of the motor 40.

The induced voltage value calculation circuit 85 acquires the inducedvoltage values eu, ev, and ew of the respective phases that arecalculated by the phase induced voltage value calculation circuit 84 anda previous value of the estimated electrical angle θmb (value calculatedearlier by one calculation period). By using the previous value of theestimated electrical angle θmb, the induced voltage value calculationcircuit 85 converts the induced voltage values eu, ev, and ew of therespective phases to induced voltage values (ed and eq) that aretwo-phase vector components in the d/q coordinate system. The inducedvoltage value calculation circuit 85 calculates, as an induced voltagevalue (absolute value) E, a square root of the sum of squares of thetwo-phase induced voltage values (ed and eq).

The angular velocity calculation circuit 86 acquires the induced voltagevalue E calculated by the induced voltage value calculation circuit 85.The angular velocity calculation circuit 86 calculates an estimatedangular velocity ωe based on the induced voltage value E. The estimatedangular velocity ωe is an estimated value of an angular velocity of themotor 40 that is a change amount of the electrical angle θma of themotor 40 per unit time. There is a proportional relationship between theinduced voltage value E and the estimated angular velocity ωe.Therefore, the estimated angular velocity ωe is obtained by dividing theinduced voltage value E by a predefined induced voltage constant(counter-electromotive force constant).

Since the motor 40 is coupled to the steering shaft 11 via the speedreducing mechanism 42, there is a correlation between the electricalangle θma of the motor 40 and a steering angle θs that is a rotationangle of the steering wheel 10 (steering shaft 11). Therefore, thesteering angle θs can be calculated based on the electrical angle θma ofthe motor 40. There is also a correlation between the angular velocityof the motor 40 and a steering speed (ωs) that is a change amount of thesteering angle θs of the steering wheel 10 per unit time.

The first estimated electrical angle calculation circuit 80 acquires thesteering torque Trq and the estimated angular velocity ωe calculated bythe angular velocity calculation circuit 86. The first estimatedelectrical angle calculation circuit 80 calculates a first additionangle Δθm1 based on the estimated angular velocity ωe. The firstaddition angle Δθm1 is a change amount of the estimated electrical angleθmb in one calculation period. The first estimated electrical anglecalculation circuit 80 calculates the first addition angle Δθm1 bymultiplying the estimated angular velocity ωe by the control period. Thefirst estimated electrical angle calculation circuit 80 sets thepositive or negative sign of the value of the first addition angle Δθm1while the positive or negative sign of the value of the steering torqueTrq is assumed to be a rotation direction of the motor 40.

The second estimated electrical angle calculation circuit 81 acquiresthe steering torque Trq. The second estimated electrical anglecalculation circuit 81 calculates a second addition angle Δθm2 based onthe steering torque Trq. The second addition angle Δθm2 is a changeamount of the estimated electrical angle θmb in one calculation period.The second estimated electrical angle calculation circuit 81 sets thepositive or negative sign of the value of the second addition angle Δθm2based on the positive or negative sign of the value of the steeringtorque Trq. The second estimated electrical angle calculation circuit 81is described later in detail.

The switching circuit 82 acquires the first addition angle Δθm1, thesecond addition angle Δθm2, and the induced voltage value E. Theswitching circuit 82 switches the addition angle to be supplied to theintegration circuit 83 between the first addition angle Δθm1 and thesecond addition angle Δθm2 through comparison between the inducedvoltage value E and a threshold voltage (positive value). When theinduced voltage value E is higher than the threshold voltage, theswitching circuit 82 supplies the first addition angle Δθm1 to theintegration circuit 83. When the induced voltage value E is equal to orlower than the threshold voltage, the switching circuit 82 supplies thesecond addition angle Δθm2 to the integration circuit 83.

The threshold voltage is set from the viewpoint of whether a deviationof the estimated electrical angle θmb calculated based on the inducedvoltage value E is a value that falls within a permissible rangerequired based on product specifications or the like, in other words,whether a calculation accuracy of the estimated electrical angle θmbthat is required based on product specifications or the like can besecured.

There are correlations between the induced voltage value E and theangular velocity of the motor 40 and between the angular velocity of themotor 40 and the steering speed. Therefore, the induced voltage value Eincreases as the steering speed increases. Conversely, the inducedvoltage value E decreases as the steering speed decreases. Thus, theinduced voltage value E is equal to or lower than the threshold voltagein a situation in which the steering speed is lower. In this situation,noise included in the detection value of each sensor (53 u to 53 w or 54u to 54 w) is likely to have a significant influence. Therefore, it isdifficult to secure the calculation accuracy of the estimated electricalangle θmb. In a situation in which the angular velocity of the motor 40and furthermore the steering speed are higher, the induced voltage valueE is higher than the threshold voltage. In this situation, thecalculation accuracy of the estimated electrical angle θmb can besecured.

The integration circuit 83 acquires the first addition angle Δθm1 or thesecond addition angle 401112 that is supplied from the switching circuit82. The integration circuit 83 includes a storage circuit 83 aconfigured to store the previous value of the estimated electrical angleθmb (value obtained earlier by one calculation period). The integrationcircuit 83 calculates the estimated electrical angle θmb by integratingthe first addition angle Δθm1 or the second addition angle Δθm2 with theprevious value of the estimated electrical angle θmb that is stored inthe storage circuit 83 a.

Thus, the rotation angle estimation circuit 77 calculates the estimatedelectrical angle θmb based on the induced voltage value E in a situationin which the calculation accuracy of the estimated electrical angle θmbcan be secured (induced voltage value E>threshold voltage). That is, therotation angle estimation circuit 77 calculates the estimated electricalangle θmb by integrating the first addition angle Δθm1 calculated by thefirst estimated electrical angle calculation circuit 80. In a situationin which the calculation accuracy of the estimated electrical angle θmbcannot be secured (induced voltage value E≤threshold voltage), therotation angle estimation circuit 77 calculates the estimated electricalangle θmb based on the steering torque Trq in place of the inducedvoltage value E. That is, the rotation angle estimation circuit 77calculates the estimated electrical angle θmb by integrating the secondaddition angle Δθm2 calculated by the second estimated electrical anglecalculation circuit 81.

When the rotation angle sensor-less control for controlling the motor 40by using the estimated electrical angle θmb is executed, in particular,when the induced voltage value E is lower than the threshold voltage,there may be a demand to further improve the response of steeringassistance to driver's steering.

In this example, the second estimated electrical angle calculationcircuit 81 is configured as follows. As illustrated in FIG. 5, thesecond estimated electrical angle calculation circuit 81 includes afirst calculation circuit 91, a second calculation circuit 92, adistribution calculation circuit 93, and a differentiator 94.

The differentiator 94 calculates an induced voltage derivative dE bydifferentiating the induced voltage value E. The phase of the inducedvoltage derivative dE is advanced relative to that of the inducedvoltage value E. The first calculation circuit 91 calculates a firstpre-addition angle β1 based on the steering torque Trq. The firstcalculation circuit 91 has a first map M1, and calculates the firstpre-addition angle β1 by using the first map M1.

As illustrated in a graph of FIG. 6, the first map M1 has a horizontalaxis that represents the steering torque Trq and a vertical axis thatrepresents the first pre-addition angle β1, and defines a relationshipbetween the steering torque Trq and the first pre-addition angle β1. Thefirst map M1 has the following characteristics. That is, when theabsolute value of the steering torque Trq is equal to or higher than afirst threshold Trq1, the absolute value of the first pre-addition angleβ1 increases as the absolute value of the steering torque Trq increases,and the absolute value of the first pre-addition angle β1 is kept at aconstant value from a second threshold Trq2 (>Trq1). The positive ornegative sign of the first pre-addition angle β1 is identical to thepositive or negative sign of the steering torque Trq. A dead band isprovided in a range near a point where the absolute value of thesteering torque Trq is zero (to be more exact, lower than the firstthreshold Trq1). In the dead band, the value of the first pre-additionangle β1 is zero.

As illustrated in FIG. 5, the second calculation circuit 92 calculates asecond pre-addition angle β2 based on the induced voltage derivative dE.The second calculation circuit 92 has a second map M2, and calculatesthe second pre-addition angle β2 by using the second map M2.

As illustrated in a graph of FIG. 7, the second map M2 has a horizontalaxis that represents the induced voltage derivative dE and a verticalaxis that represents the second pre-addition angle β2, and defines arelationship between the induced voltage derivative dE and the secondpre-addition angle β2. The second map M2 has the followingcharacteristics. That is, when the absolute value of the induced voltagederivative dE is equal to or higher than a first threshold dE1, theabsolute value of the second pre-addition angle β2 increases as theabsolute value of the induced voltage derivative dE increases, and theabsolute value of the second pre-addition angle β2 is kept at a constantvalue from a second threshold dE2 (>dE1). The positive or negative signof the second pre-addition angle β2 is identical to the positive ornegative sign of the induced voltage derivative dE. A dead band isprovided in a range near a point where the absolute value of the inducedvoltage derivative dE is zero (to be more exact, lower than the firstthreshold dE1). In the dead band, the value of the second pre-additionangle β2 is zero.

As illustrated in FIG. 5, the distribution calculation circuit 93determines use ratios of the first pre-addition angle β1 and the secondpre-addition angle β2 based on the induced voltage value E, andcalculates the second addition angle Δθm2 based on the determined useratios.

The distribution calculation circuit 93 has a third map M3, andcalculates a distribution gain G by using the third map M3. Thedistribution gain G is used for determining the use ratios of the firstpre-addition angle β1 and the second pre-addition angle β2.

As illustrated in a graph of FIG. 8, the third map M3 has a horizontalaxis that represents the induced voltage value E and a vertical axisthat represents the distribution gain G, and defines a relationshipbetween the induced voltage value E and the distribution gain G. Thethird map M3 has the following characteristics. That is, thedistribution gain G is set to a higher value as the induced voltagevalue E increases. Conversely, the distribution gain G is set to a lowervalue as the induced voltage value E decreases. The distribution gain Gis a value that falls within a range of “0” to “1”.

The distribution calculation circuit 93 calculates the second additionangle Δθm2 by applying the distribution gain G to Expression (A).Δθm2=β1·G+β2·(1−G)  (A)

In Expression (A), the symbol “G” represents the distribution gain, andalso represents the use ratio of the first pre-addition angle β1. Thesymbol “1-G” represents the use ratio of the second pre-addition angleβ2.

In Expression (A), the distribution gain G is set to a value from “0” to“1”. When the distribution gain G is “0”, the use ratio of the secondpre-addition angle β2 is 100%. When the distribution gain G is “1”, theuse ratio of the first pre-addition angle β1 is 100%. When thedistribution gain G is a value between “1” and “0”, the firstpre-addition angle β1 and the second pre-addition angle β2 are addedtogether at use ratios that are based on the value of the distributiongain G. In this manner, the use ratios of the first pre-addition angleβ1 and the second pre-addition angle β2 are adjusted based on the valueof the distribution gain G.

As illustrated in the graph of FIG. 8, the distribution gain G is set toa higher value as the induced voltage value E increases. Therefore, asthe induced voltage value E increases, the use ratio of the firstpre-addition angle β1 increases, and the use ratio of the secondpre-addition angle β2 decreases. Conversely, the distribution gain G isset to a lower value as the induced voltage value E decreases.Therefore, as the induced voltage value E decreases, the use ratio ofthe second pre-addition angle β2 increases, and the use ratio of thefirst pre-addition angle β1 decreases.

According to this embodiment, the following actions and effects can beattained.

(1) When the rotation angle sensor-less control is executed and when theinduced voltage value E is equal to or lower than the threshold voltage,it is difficult to secure the calculation accuracy of the inducedvoltage value E. Therefore, the driving of the motor 40 is controlledbased on the second addition angle Δθm2 in place of the first additionangle Δθm1 that is based on the induced voltage value E. The phase ofthe second addition angle Δθm2 is compensated based on the derivative ofthe induced voltage value E that is a steering condition amount (statevariable) that reflects the steering condition. That is, as representedby Expression (A), calculation is executed such that values obtained bymultiplying the first pre-addition angle β1 calculated based on thesteering torque Trq and the second pre-addition angle β2 calculatedbased on the induced voltage derivative dE by respective predetermineduse ratios that are based on the induced voltage value E are addedtogether. The phase of the induced voltage derivative dE is advancedrelative to that of the induced voltage value E. The phase of the secondaddition angle Δθm2 is advanced as a whole by adding, to the firstpre-addition angle β1, the second pre-addition angle β2 that is based onthe induced voltage derivative dE. By using the second addition angleΔθm2 obtained after the phase compensation, a more appropriate estimatedelectrical angle θmb is calculated in response to the driver's steeringsituation. Thus, the response of the steering assistance to the driver'ssteering can be improved. Furthermore, a sense of friction can bereduced during the driver's steering.

(2) The distribution calculation circuit 93 sets the use ratio of thesecond pre-addition angle β2 that is based on the induced voltagederivative dE to a higher value as the induced voltage value Edecreases. Therefore, a more appropriate second addition angle Δθm2 isobtained in response to the steering condition.

(3) The steering control apparatus 50 includes the rotation angleselection circuit 79. The rotation angle selection circuit 79 selects,as the electrical angle of the motor 40 to be used for controlling powersupply to the motor 40, one of the electrical angle θma detected throughthe rotation angle sensor 61 provided on the motor 40 and the estimatedelectrical angle θmb calculated by the integration circuit 83. Therotation angle selection circuit 79 selects the electrical angle θmadetected through the rotation angle sensor 61 when an abnormality of therotation angle sensor 61 is not detected, and selects the estimatedelectrical angle θmb calculated by the integration circuit 83 when theabnormality of the rotation angle sensor 61 is detected. According tothis configuration, the power supply to the motor 40 can be controlledcontinuously by using the estimated electrical angle θmb even when theabnormality occurs in the rotation angle sensor 61.

(4) The second estimated electrical angle calculation circuit 81 has thefirst map M1 that defines the relationship between the steering torqueTrq and the first pre-addition angle β1, and the second map M2 thatdefines the relationship between the induced voltage derivative dE andthe second pre-addition angle β2. The second estimated electrical anglecalculation circuit 81 can easily calculate the first pre-addition angleβ1 by using the first map M1, and the second pre-addition angle β2 byusing the second map M2. The second estimated electrical anglecalculation circuit 81 also has the third map M3 that defines therelationship between the induced voltage value E and the distributiongain G. The second estimated electrical angle calculation circuit 81 caneasily calculate the distribution gain G by using the third map M3.

(5) In the first map M1, when the absolute value of the steering torqueTrq is lower than the first threshold Trq1, the value of the firstpre-addition angle β1 is set to zero. The dead band provided in thismanner suppresses influence of an event that the positive value and thenegative value of the steering torque Trq are switched frequently andrepeatedly near zero, that is, an event that the positive value and thenegative value of the first pre-addition angle β1 are switchedfrequently and repeatedly. Thus, the calculation accuracy of the firstpre-addition angle β1 can be secured.

(6) In the second map M2, when the induced voltage derivative dE islower than the first threshold dE1, the value of the second pre-additionangle β2 is set to zero. The dead band provided in this mannersuppresses influence of an event that the positive value and thenegative value of the induced voltage derivative dE are switchedfrequently and repeatedly near zero due to influence of noise or thelike, that is, an event that the positive value and the negative valueof the second pre-addition angle β2 are switched frequently andrepeatedly. Thus, the calculation accuracy of the second pre-additionangle β2 can be secured.

This embodiment may be modified as follows. There may be employed aconfiguration in which the dead bands are omitted from the first map M1and the second map M2.

As indicated by parenthesized symbols in FIG. 5, the second estimatedelectrical angle calculation circuit 81 may calculate the secondpre-addition angle β2 by using a steering torque derivative dTrqobtained by differentiating the steering torque Trq in place of theinduced voltage derivative dE. In this case, as indicated by aparenthesized symbol in FIG. 7, a map that defines a relationshipbetween the steering torque derivative dTrq (absolute value) and thesecond pre-addition angle β2 is used as the second map M2. When thesteering torque Trq changes, the second pre-addition angle β2 ispromptly calculated based on the steering torque derivative dTrq. Inparticular, the steering torque derivative dTrq is preferably used whenan attempt is made to achieve improvement (reduction) in terms of thesense of friction on the steering torque Trq. By reducing the sense offriction, the response of the steering assistance is further increased.

The second estimated electrical angle calculation circuit 81 maycalculate the second pre-addition angle β2 by using both the inducedvoltage derivative dE and the steering torque derivative dTrq. Also inthis case, the second addition angle Δθm2 is calculated such that apre-addition angle that is based on the steering torque Trq, apre-addition angle that is based on the induced voltage derivative dE,and a pre-addition angle that is based on the steering torque derivativedTrq are added together at predetermined use ratios. With thisconfiguration, the response of the steering assistance to the driver'ssteering can be improved, and the sense of friction on the steeringtorque Trq can be reduced.

The first estimated electrical angle calculation circuit 80 calculatesthe first addition angle Δθm1 based on the estimated angular velocity ωeobtained from the induced voltage value E, but may calculate the firstaddition angle Δθm1 based also on the induced voltage derivative dE.Specifically, the first estimated electrical angle calculation circuit80 has a configuration similar to that of the second estimatedelectrical angle calculation circuit 81 illustrated in FIG. 5. That is,the first addition angle Δθm1 is calculated such that values obtained bymultiplying a first pre-addition angle (β1) that is based on theestimated angular velocity ωe (induced voltage value E) and a secondpre-addition angle (β2) that is based on the induced voltage derivativedE by respective use ratios set based on the induced voltage value E areadded together.

In this example, the rotation angle sensor-less control for controllingthe motor 40 based on the estimated electrical angle θmb is executed asbackup control for the case where an abnormality occurs in the rotationangle sensor 61. The rotation angle sensor-less control need not beexecuted as the backup control, but the motor 40 may constantly becontrolled without using the rotation angle sensor 61. In this case, therotation angle sensor 61 may be omitted.

In this example, the steering control apparatus 50 is applied to theelectric power steering system of the type in which the assist force isapplied to the steering shaft 11 (column shaft 11 a). The steeringcontrol apparatus 50 may be applied to an electric power steering systemof a type in which the assist force is applied to the rack shaft 12. Inthis case, the torque sensor 60 may be provided on the pinion shaft 11c.

What is claimed is:
 1. A steering control apparatus configured tocalculate a current command value for a motor based on at least asteering torque, calculate an estimated electrical angle of the motorbased on an induced voltage generated in the motor, and control powersupply to the motor by using the calculated estimated electrical angle,the motor being a source of power to be applied to a steering mechanismof a vehicle, the steering control apparatus comprising: a firstestimated electrical angle calculation circuit configured to calculate,based on the induced voltage, a first addition angle that is a changeamount of the estimated electrical angle in one calculation period; asecond estimated electrical angle calculation circuit configured tocalculate, based on the steering torque, a second addition angle that isa change amount of the estimated electrical angle in one calculationperiod; a selection circuit configured to select the first additionangle when the induced voltage is higher than a threshold voltage, andselect the second addition angle when the induced voltage is equal to orlower than the threshold voltage; and an integration circuit configuredto calculate the estimated electrical angle by integrating the firstaddition angle or the second addition angle that is selected by theselection circuit, wherein the second estimated electrical anglecalculation circuit is configured to compensate a phase of the secondaddition angle based on a derivative of a steering condition amount thatreflects a steering condition.
 2. The steering control apparatusaccording to claim 1, wherein the second estimated electrical anglecalculation circuit includes: a first calculation circuit configured tocalculate, based on the steering torque, a first pre-addition angle thatis a change amount of the estimated electrical angle in one calculationperiod; a second calculation circuit configured to calculate, based onthe derivative of the steering condition amount, a second pre-additionangle that is a change amount of the estimated electrical angle in onecalculation period; and a distribution calculation circuit configured tocalculate the second addition angle obtained by multiplying the firstpre-addition angle and the second pre-addition angle by respective useratios set based on the steering condition amount and adding themultiplied first pre-addition angle and the multiplied secondpre-addition angle together as processing for compensating the phase ofthe second addition angle.
 3. The steering control apparatus accordingto claim 2, wherein the second estimated electrical angle calculationcircuit has: a first map that defines a relationship between thesteering torque and the first pre-addition angle; a second map thatdefines a relationship between the steering condition amount and thesecond pre-addition angle; and a third map to be used for calculating adistribution gain for determining the use ratios of the firstpre-addition angle and the second pre-addition angle based on thesteering condition amount.
 4. The steering control apparatus accordingto claim 1, further comprising a rotation angle selection circuitconfigured to select, as an electrical angle of the motor to be used forcontrolling the power supply to the motor, one of an electrical angledetected through a rotation angle sensor provided on the motor and theestimated electrical angle calculated by the integration circuit,wherein the rotation angle selection circuit is configured to select theelectrical angle detected through the rotation angle sensor when anabnormality of the rotation angle sensor is not detected, and select theestimated electrical angle calculated by the integration circuit whenthe abnormality of the rotation angle sensor is detected.
 5. Thesteering control apparatus according to claim 1, wherein the steeringcondition amount is at least one of the induced voltage and the steeringtorque.
 6. The steering control apparatus according to claim 3, whereina dead band is set in a predetermined range including zero in each ofthe first map and the second map.